Such apparatus require driving and support or backup rollers contacting the vehicle wheels to be tested. Typically, one slippage member, adjustable in its slippage characteristic, is assigned to the powered driving rollers for changing the driving torque between a vehicle and driving rollers dependent on the applied braking torque while maintaining the adherence or grip conditions between the vehicle wheel and the driving roller. Brake test rigs of this type are known for operational testing of vehicle brake installations with antiskid system, where the slippage between the vehicle wheel and the power drive is caused by actuating the vehicle brake. In particular, the driving torque of the driving roller acting upon the vehicle is changed as a function of the braking torque acting upon the vehicle wheel for simulating slippage, without changing the adherence or grip conditions between the vehicle wheel and driving roller. The change of the driving torque with application of the braking torque corresponds essentially to predeterminable adherence and slippage conditions between the vehicle wheel and the roadway. However, instead of actual slippage between the vehicle wheel and the driving roller, the slippage is simulated in the drive train. If an inadequate drive torque at the vehicle wheel is opposed to the braking torque, there occurs an rpm drop at the vehicle wheel, which corresponds to the rpm drop to which the response of the antiskid braking system is designed. By means of the slippage member, a slippage characteristic for different roadway conditions, as, for instance, dry roadway, wet roadway, iced-up roadway, etc. can be realized, so that it is possible to simulate on the test apparatus the braking actions without any wear and reproducibly as well as independently of external influences on the respective roadway surfaces. The precondition for this is that, when applying the braking torque, the driving torque in the rpm ranges to be investigated has to have the same behavior as the empirically determined slippage characteristics for the respective roadway surfaces.
In the known brake test rig for vehicles with antiskid brake systems, a driving roller with its own drive motor is provided for each vehicle wheel. This makes the entire installation expensive. In particular, the apparatus for slippage-dependent change of the driving torque can be formed by using an electrical induction motor as a drive motor. However, the possibility also exists to arrange a controllable magnetic particle clutch as a slippage member with adjustable slippage characteristics between the drive motor and the driving roller as the apparatus changing the drive torque as a function of the braking torque. These measures, however, further increase the cost of the entire installation.